1. Field of the Invention
The present invention relates to a power tool driven by a brushless motor and, more particularly, to a technique that can maximize the output efficiency of the brushless motor in relation to the operation of the power tool.
2. Description of the Related Art
In tightening screws by utilizing a screwdriver, two types of operations as shown in FIGS. 8 and 9 are known. The operation type as shown in FIG. 8 is referred to as “hard joint” operation. To the contrary, the operation type as shown in FIG. 9 is referred to as “soft joint” operation. During the hard joint operation, the tool bit only rotates by a relatively small angle until the tightening operation is completed after the tool bit has contacted the work-piece. On the other hand, during the soft joint operation, tool bit rotates by a relatively large angle (the tool bit turns twice or more) until the tightening operation is completed.
The rotational angle of the tool bit during the hard joint operation is different from the rotational angle during the soft joint operation even if the power tool has the same torque condition for the both joints. As a result, the time required for continuously generating tightening torque until completion of the screw tightening operation becomes different between the hard joint operation and the soft joint operation. When the hard joint operation is selected, because the time required for tightening screws becomes relatively short, the inertia force of the rotating rotor can be additionally utilized for tightening the screw. On the other hand, when the soft joint operation is selected, time required for tightening the screw takes relatively long, and therefore, it is required to achieve stable tightening operation solely by means of the output torque of the motor without utilizing the inertia force of the rotor. As a result, energy efficiency to procure big torque in tightening screws should be maximized. Moreover, the output torque of the motor should be stabilized regardless of the type of operation to tighten the screw.